Role Of GABA_B Receptors In Pain Regulation At The Spinal Level

γ-aminobutyric acid (GABA) is an important inhibitory neurotransmitter in nervous system and carries out inhibitory function by binding to GABA_A, GABA_B, GABA_C receptors. GABA_B receptors is a GTP-binding protein-coupled receptor (GPCR) consisting of GABA_BR1 and GABA_BR₂ subunits for the signal transduction.Physiological data showed that fine afferents (C and A_δ fibers) can be depolarized to elicit pain and release CGRP, SP or Glu. But the process can be inhibited by activating the presynaptic GABA_B receptors on the primary afferent fibers by the GABAergic interneurons in spinal dorsal horn regulating pain transmission. However, neither the characteristics of the afferent fibers that express GABA_B receptor nor the relationship between peptidergic and nonpeptidergic afferent fibers with GABAergic neurons in spinal dorsal horn are elucidated.Astrocytes have been demonstrated to be involved in pain regulation at the spinal level, they can also express GABA_B receptors. To further understand the role of GABA_B receptors in inflammatory pain transmission, we have investigated the phosphorylation and expression changes of GABA_B receptors in astrocytes.In the present study, we first investigated the neuronal subpopulations in the DRG which express the GABA_BRI by doublefluorescence immunohistochemistry. 94% of the peptidergic and 88% of the nonpeptidergic small size DRG neurons expressed GABA_BR.In the next step, we found CGRP and IB4 immunoreactive primary afferent terminals had close contacts with the GABA positive labeled interneurons in the spinal dorsal horn by doublefluorescence immunohistochemistry. Furthermore, some central terminals of type I and type II synaptic glomeruli were labeled by GABA_BR1in the superficial spinal dorsal horn as observed by electron microscopy. We also found some astrocytes labeled by GABAbR1 in the spinal dorsal horn. From these results we suggest that presynaptic GABA_BR may be involved in the modulation of nociceptive transmission.In the third part, we studied the phosphorylation of transcription factor cAMP response element binding protein (CREB) and GABA_BR2 in cultured astrocytes treated by prostaglandin E₂ (PGE₂) at different time points by immunocytochemistry and Western blot. The levels of phosphorylation of CREB and GABA_BR2 could be enhanced significantly by PGE2 in a time-dependent manner.Furthermore, we investigated the expression changes of GABA_B receptors in astrocytes under inflammatory situation. The results showed the mRNA expression levels of GABAbR1 and GABA_BR2 were down-regulated in a time-dependent manner by PGE2. Western blot analysis showed a reduced amount of GABAbR1b protein level compared to control values by the treatment of PGE2 for 24 h.Together our observations suggest the presynaptic GABA_B receptors may be involved in the modulation of nociceptive transmission. The changes of phosphorylation and expression of GABA_B receptors reflect its critical role in CNS inflammation and pain regulation.